The DQDB protocol and a QPSX network which employs the protocol are described in the specification U.S. Pat. No. 5,157,657 (which corresponds to Australian Patent Application No. 45968/89) and the specifications of U.S. Pat. Nos. 4,922,244 and 5,051,742 derived from International Application No. PCT/AU85/00304 (International Publication No. WO 86/03639). The U.S. patent specifications are herein incorporated by reference. A discussion of the DQDB protocol and a QPSX network are also provided in the report prepared by the IEEE 802/6 Working Group entitled "Proposed IEEE Standard 802.6--Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)" Draft D10, May 1989, and the International Journal of Digital and Analog Cabled Systems, Vol. 2 179-186 (1989).
In communications networks which employ the DQDB communications protocol, slots which are used to transmit data from one station of the network to another cannot be reused by subsequent stations, along the bus on which the data is transmitted, once the slot has been read by the destination station. Empty slots which can be used for data transmission between stations are only provided by Head of Bus (HOB) units of the network. This is also the case for networks which employ an improved DQDB protocol which enables guaranteed capacity to be allocated to stations in the network. The improved protocol is described in A. Kennington, Guaranteed Bandwidth in Distributed Queuing, Contributions 802.6-90/50-51 to the IEEE 802.6 Working Group, July 1990. Stations which operate on the basis of the basic DQDB protocol are hereinafter referred to as DQDB stations, and stations which operate according to the improved DQDB protocol and are able to provide guaranteed capacity for a priority level are hereinafter referred to as GBW stations.
To increase the capacity of a network which employs a DQDB protocol it is desirable to provide a method and a station for efficiently erasing slots after they have been received by all destinations, specified by their address field, so they can be used again before proceeding to an End of Bus (EOB) unit. Erasing a slot entails resetting the busy bit in the slot's header and, if desired, resetting the information carrying bits of the slot so it may be used by another station to transmit data to a succeeding station. A distributed queue established between stations of the network however should be maintained if slots are erased and made available for use again before reaching an EOB unit.